Light gathering in M43 vs FF

So there's a lot of discussions regarding FOV of M43 vs FF, and I think I have a good grasp on that aspect.

Thought one thing that bugs me is light gathering. Let me ask the question with a simple example:

Suppose I have two camera setups:

• 25mm f1.4 lens on a M43
• 50mm f1.4 lens on a FF

Same scene, same placement, shot wide open at say ISO 200. Assume that processing between the cameras is exactly the same, I set the exposure to 1/100, will I get the same image in terms of amount of light gathered?

I believe the answer here is yes.  The only difference in this theoretical set up should be the DOF.

The amount of light gathered is 4X on the FF sensor in this case, which will lead to a two stop idealized improvement (the ISO 200 on FF will look like an ISO 50 m4/3 shot, all else equal). The exposure will be the same for both cameras.

What is your understanding of light gathering?

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Thanks pgilland, understandably the DOF will be different. From a DOF perspective 25mm f1.4 on M43 will be equivalent to 50mm f2.8 on FF.

I was simply thinking about (trying to understand) scenarios where you are shooting sports or nature, where it's more about exposure than DOF. Since, if we're talking about super-telephoto, most images on M43 will have enough subject isolation (or enough background blur) if you are zoomed in on your subject where getting more isolation (background blur) benefit from FF becomes irrelevant.

In that case, if you can get the same shot with 200mm f2.8 on M43 vs. 400mm f2.8 on FF with the same exposure, then you're really benefiting a lot from M43 format from a space/weight savings perspective.

The important thing in any electronic signal capture--regardless of whether we're talking about photography, or audio recording, or any number of other things--is signal to noise ratio. It's simple: the more signal you have going into your end result, the less noise there is.

For your parameters, you get the same amount of light per unit of area, which maintains the same exposure. However, due to the differences in total area of the sensor, the FF setup gathers about four times as much light, meaning you have four times as much signal going into the image. Which means the noise that's present is less evident, by a factor of about two stops, for sensors of similar quantum efficiency.

You also get half the depth of field on the FF image.

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Not sure how to put it, I guess luminosity? Basically, with the two setups, assume one is exposed correctly to get a nicely lit image, the other will have the same.

This is exactly what I wanted to get clarity on, but I think if you mean light gathered is 4X, then you should mean ISO 50 on FF and ISO 200 on M43?

You will see some slight variations, but if you set your FF camera to F/1.4, ISO100 and 1/60th shutter speed, for the same exposure on an APS, M43, Nikon1, etc, you will need F/1.4, ISO100 and 1/60th.

Ignore the arguments about noise for now because that is a different subject.

I do understand a bit about signal to noise, I think "light gathering" is a poor choice of words. Thanks for the clarification.

Both cameras will show identical exposure ( shutter/ aperture)

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per unit area.

That is why an incidence light meter doesn't ask you what format film or camera you are shooting with-same exposure regardless of format.

Did that help?

Didn't think so.

Why don't you just ask what you are really concerned about and we will help you.

TEdolph

Basically it sounds like some of the other responses here are saying the same thing I said initially, only in rather complex terms.

Yes, FF lens might have 4x the light gathering of 4/3 lens at equal f stop  but that's only because it has more area that it needs to spread the light over.  The light for any given equal  area is the same regardless of the format.

You could start talking signal noise, but it's not relevant here.  Besides then you really have to talk what specific cameras your comparing. OMD-EM5 has come fare enough in the noise department to equal some larger format cameras at upper ISO's regardless of signal density and what not.  Bottom line, regardless of noise signal, 4/3 can produce clean results.

If you only asking about light gathering, then I think you're on the right track.

When you want to start talking about DOF, it gets a little more complex.  Longer lenses will give more DOF, especially if your up close to your subject.  Get farther away, and FF might take back some of the advantage of larger format.  I don't really go for the super shallow DOF myself (personal taste), so I don't worry about that as much.  I think your in the same boat, 4/3 and longer lenses generally provide plenty enough shallow DOF to make me happy.

When looking at DOF from the other end of the perspective, which is the way I look at it for the scenarios you suggested (sports and nature), my personal opinion is the smaller 4/3 format has a significant advantage.  With FF wide open to get high shutter speed, not just in lower light situations, but to stop motion blur, you have to sacrifice DOF and sharpness.  Having that extra DOF with 4/3 while shooting wide open for high shutter speed also means more likely to be in focus as well, and the best m4/3 lenses tend to be sharper wide open than some FF counterparts when wide open.

There is light gathering per unit area, and there is the total light gathered for the purpose of constructing the images. For the former, yes 1.4 is 1.4 regardless of sensor size and the exposure is the same given the same shutter speed. However, imagine you are going to eventually show the image in the same way, e.g. enlarged to A4 size for both systems. One A4 print will have been the result of 4x the total original quantity of light compared with the other.

The size of the pupils of the lenses are different when both are at 1.4, with the 50mm lens having a pupil four times the area as the 25mm one. So at the pupil one allows in 4x the amount of total light during the exposure, given the same shutter speed.

If your eventual display image from FF is 4x the area of that from M4/3 (same enlargement ratio in both cases) then 1.4 is 1.4 across both systems and there is no advantage/disadvantage either way.

Actually it did help.

let me ask it this way:

Why do you care?

Tedolph

In such a setup, the 135 FF format sensor will have approximately four times as many photons hitting it compared to the four-thirds sized sensor.

But as others have pointed out, the cameras aren't set up to take the same picture. For the same DoF the aperture on the FF needs to be two stops smaller than the aperture on the M43 and all other things being equal, that cancels out the FF photonic advantage.

Or you can phrase it as follows: To take the picture the FF is taking, the M4/3 lens needs to be two stops faster, which means that no photo will be taken as no such lens exists, resulting in the poorest possible showing for the latter, a camera system incapable of taken a photo

Why do you care what I care about?

By the way, may help to read the other posts in this thread, I did talk about "why I care".

Also, it seems every single one of your posts have a "+1" to the end of it, I guess you have a secret admirer!

All camera systems have limitations, sometimes it stifles creativity, sometimes it acts as a catalyst. I carry my M43 system around a lot more than I did my 135 system and the camera you have with you always takes a lot better pictures than the one you left at home.

Light and luminosity aren't the same thing. At least in what is probably your understanding of luminosity. But since you bring ISO into the equation, it should be said that ISO sets by its very definition the ratio between 'brightness' at the sensor and 'brightness' in the final image. Thus when asking whether the same exposure yields the same image brightness when keeping the ISO constant, the answer is yes, by definition (of ISO).

(And that is a simplified answer as there are several slightly different definitions of ISO.)